Inductive conductivity sensors serve in a large number of applications in laboratory and process measurements technology for registering the conductivity of a liquid medium. They are used preferably where large measuring ranges and high chemical or thermal loadings occur. This is the case, for example, in a large number of industrial chemical processes and also in hot steam sterilization methods, which are frequently applied in foods technology due to the high hygiene requirements.
As a result of the mentioned requirements, frequently high performance plastics are used. Polyetheretherketone (abbreviated PEEK) is an example of a high temperature resistant, thermoplastic, synthetic material and is a member of the group, polyaryletherketones. Polyetheretherketone (PEEK) is a partially crystalline thermoplastic with high tensile- and bending strength, high impact toughness, high fatigue strength, high chemical resistance and is only difficultly burnable.
An inductive conductivity sensor includes a transmitting coil and a receiving coil, which, as a rule, are embodied as ring coils, also referred to as toroidal coils. Such a conductivity sensor functions as a type of double transformer, wherein the transmitting and receiving coils are inserted so far into the medium that a closed electrical current path can form extending through the medium and passing through the transmitting and receiving coils. When the transmitting coil is excited with an alternating voltage signal as an input signal, it produces a magnetic field, which induces in the closed path through the medium passing through the coils an electrical current, whose strength depends on the electrical conductivity of the medium. Since this alternating electrical current in the medium brings about, in turn, a variable magnetic field surrounding it, an alternating electrical current is induced in the receiving coil as an output signal. This alternating electrical current, respectively a corresponding alternating voltage, delivered from the receiving coil as an output signal is a measure for the electrical conductivity of the medium.
For feeding the transmitting coil with an alternating voltage, an inductive conductivity sensor includes a driver circuit connected with the transmitting coil. For registering the output signal of the receiving coil, the conductivity sensor includes, moreover, electrically connected with the receiving coil, a receiving circuit, which is embodied to output the registered measurement signal, in given cases, conditioned by the receiving circuit, to a sensor electronics, which serves to process the measurement signal further and, in given cases, to digitize it. Frequently, conductivity sensors are embodied as measuring probes at least sectionally immersible in the medium. Such measuring probes include a housing, in which the transmitting and receiving coils, in given cases, the driver circuit and the receiving circuit as well as other circuit parts assembled with the transmitting and receiving circuit into a sensor circuit, are accommodated. The measuring probe is connected in such an embodiment with a removed, superordinated unit, for example, a display unit, a measurement transmitter, or a computer. The superordinated unit can be embodied both for energy supply of the measuring probe as well as also for data communication with the measuring probe. The sensor circuit optionally contained in the measuring probe can be embodied to forward the further processed, in given cases, digitized, measurement signal to the superordinated unit. The corresponding measured value can be displayed via the superordinated unit by means of a display system or output via a data interface.
The coils of the inductive conductivity sensors can be provided with a housing in various ways. In one known method, a coil component is formed, in the case of which the coils are, first of all, in a complex method, introduced into a separate coil housing and then injection molded around, in this coil housing, with plastic. The so produced assembly is then inserted into a sensor housing. The creating of the separate coil housing is necessary, in order to protect the coils in the injection molding from the high injection pressures and very high temperatures during the injection procedure. Since the coils are very sensitive to pressure and temperature, there is present in this processing besides a high preparation- and assembly effort for the coil component a high rejection risk.
In the case of a known sensor, two coils are arranged, in each case, on a side of a circuit board and then the prefabricated circuit board inserted into the sensor housing. The circuit board includes, in such case, an opening, in order to position the coils in the form of toroidal coils. After the insertion of the circuit board into the sensor housing, the housing is sealed by a sleeve, which is inserted through a wall of the housing into the housing and guided through the opening of the circuit board. The two ends of the sleeve are, in such case, adhered to the housing at the interfaces with such. The inner space of the sensor housing is, in this connection, filled with a potting compound. Besides the preparation effort for the adhesive locations, which must be cleaned before the adhesion process, also a rework of the adhesive at the transition locations is necessary. The adhesive gap, which forms between sleeve and housing, has a different coefficient of thermal expansion than the housing, wherein in the case of temperature changes during use of the inductive sensor the adhesive gap can be damaged. Moreover, the adhesive has a lesser chemical durability than the plastic. For manufacturing the two separate adhesive locations, a complex handling is necessary, which lengthens the time required for the procedure.
Known from German Patent DE 10 2010 042 832 is a process utilizing ultrasonic welding.
The dissertation of Joachim Nehr with the title “Neuro-Fuzzy-Modellierung zur umfassenden Prozessüberwachung am Beispiel des Ultraschallschweiβens von Kunststoffteilen (neuro-fuzzy modeling for comprehensive process monitoring in the example of ultrasonic welding of plastic parts)”, Universitat Stuttgart (University of Stuttgart), year 2011, describes the ultrasonic welding process in the following way: An oscillatable system, composed of converter, booster and sonotrode, is caused to oscillate with longitudinal oscillations in the ultrasonic region by excitation by means of piezo elements in the converter. Typical oscillation frequencies are, in such case, 20, 30, 35 or 40 kHz, depending on machine manufacturer and size of the workpieces to be welded. The initial oscillation amplitude in the range from about 6 to 13 μm—depending on oscillation frequency—is amplified by the booster (also called the amplitude transformer) and the sonotrode application- and material specifically by a factor in the range, 1-5. By mechanical coupling of the sonotrode, the oscillation is introduced into the workpiece. The oscillatory energy is absorbed by inner friction and boundary friction, whereupon the occurring heat leads to a local melting in the joint zone and thereby to connection of the two joint partners.
The process flow in the case of ultrasonic welding is divided into three phases: 1) The start, wherein the sonotrode acts on the components to be connected and the mechanical coupling occurs with a selectable force; 2) The actual welding phase, during which the oscillation produced by the converter is introduced into the component at a given force level and leads to melting and connecting of the components; and 3) The holding phase for cooling the melt arising in the welding until the solid weld seam forms.
Depending on separation between weld zone and sonotrode where the oscillation is being introduced, a distinction is drawn between near field and far field welding, wherein a separation of about 6 mm marks the border between the two types of welding. Far field welds can be performed best with stiff thermoplastics, since an upturning of the parts related to the welding force should be kept as small as possible or completely prevented. Due to the shear modulus and mechanical loss factor, most often, only amorphous synthetic materials are welded in the far field. Partially crystalline plastic materials, which, most often, have a marked damping rise already well below the melting temperature, should only be welded in the near field.